[Uncle Peter]

There's always the possibility that the USAF are operating their new toys in Iranian airspace with impunity. Stealthy aircraft aren't completely invisible to search radar so if a missile was fired then it could have been at a "legitimate" target. This would inevitably end very badly if the air defence system didn't have the ability to actually track the target.

Imagine the loss of face if the Iranians had to admit that they cannot defend their own airspace against enemy reconnaissance assets.